Mice with the p53 tumor suppressor gene knocked out by gene targeting develop normally but have increased susceptibility to spontaneous tumor development. We are characterizing and using nullizygous p53-knockout mice as a model of genetic susceptibility to screen for nutritional and chemopreventive agents which can offset the increased risk of tumorigenesis resulting from the loss of tumor suppressor function. We have shown that calorie restriction (CR; a potent inhibitor of many types of rodent tumors) delays spontaneous tumor development in p53-knockout mice. We have also assessed the effect of the putative chemopreventive agents dehydroepiandrosterone (DHEA), quercetin, d-limonene and all-trans retinoic acid on spontaneous tumorigenesis in p53-knockout mice to further establish the efficacy of this model for testing cancer prevention strategies. DHEA, an inhibitor of glucose 6-phosphate dehydrogenase, impressively delays tumor development in p53-knockout mice, while the other agents have no significant effects. These studies established that p53-knockout mice provide a useful in vivo model of genetic susceptibility since tumorigenesis in these mice is spontaneous, rapid, relevant to human cancer and responsive to experimental manipulation. Currently in progress are molecular studies on tissues collected serially from control, CR- and DHEA-treated wild-type and p53-knockout mice. We are analyzing tissues for differences in p53, WAF/p21, ras, bcl-2, myc, cdc-2, and mdr expression using Northern blot analysis and immunohistochemistry. We are also comparing differential messenger RNA expression in liver samples from these mice using the differential display reverse transcription-PCR technique. Other studies in progress include: 1) a pathological/immunological/virological characterization of tumors from p53- knockout mice; 2) an analysis of tumor development in p53-knockout mice in response to DHEA analogues to further characterize this class of chemopreventive compounds; 3) a 2-year study of CR effects on spontaneous tumorigenesis in wild-type mice which is providing evidence that CR modulates tumorigenesis through both p53-dependent and p53-independent mechanisms; 4) a study of CR effects on 2-stage skin carcinogenesis and inflammation/oxidant production in male hemizygous p53-knockout mice and 5) a comparison of the effects of different doses of chronic and acute nitrosomethylurea exposure in female hemizygous p53-knockout mice, testing the sensitivity of these mice to chemically-induced tumor development; 6) an assessment of the effects of CR on nitrosomethylurea-induced tumor development and mutation rates.